Metal wires for reinforcing tire elastomers and suitable for use in the field of making parts exposed to fatigue must have a small diameter, in general between 0.1 mm and 0.4 mm, and high-performance mechanical characteristics. The tensile strength can be higher than 2200 MPa, the residual ductility, as measured by the reduction of area in tension, torsion or by wrap-around test must be non-zero, the fatigue endurance limit under rotational or alternating bending stress must be greater than 1000 MPa.
These characteristics are necessary to withstand the static or alternating forces to which the wire is subjected in the assemblies incorporated in tires.
In addition, drawing of the stainless-steel wire to a diameter of between 0.1 and 0.4 mm must be possible under industrial conditions, or in other words with the lowest possible incidence of breaking.
French Patent Application 93-12528 relates to the use of a stainless-steel wire of diameter of between 0.05 mm and 0.5 mm with tensile strength Rm higher than 2000 MPa. The steel of which the wire is made contains in its composition at least 50% martensite obtained by drawing to a reduction ratio of larger than 2.11 with intermediate annealing treatments, the total nickel plus chromium content being between 20% and 35%.
Patent No. 97-01858 relates to the forming of an austenitic stainless-steel wire in the state of work-hardened drawn wire containing a certain proportion of martensite formed during drawing, the drawing being performed without annealing, with a cumulative reduction ratio of greater than 6.
Cumulative deformation by drawing .epsilon. is understood as the value of the Naperian logarithm of the ratio of the initial and final cross sections (.epsilon.=log [So/Sf]).
The described process specifies compositions which are particularly stable with respect to the martensite produced by work hardening, thus permitting tensile strengths of higher than 2200 MPa to be achieved when the cumulative deformation is extremely high and above 6.
Tire reinforcements are generally made by stranding wires of diameter between 0.1 mm and 0.30 mm. In the case of stainless steels, a tensile strength of 2200 MPa is sufficient in view of the fact that the behavior of the steel in service is not degraded or is degraded only slightly by the moist environment.
Higher tensile strengths may be of industrial interest, but difficulties are encountered in forming wires with very high-performance mechanical strength characteristics by drawing, because they become break-sensitive, in particular due to an excess of martensite.
It may be useful to propose, to the manufacturers of tire reinforcements, steel wires capable of being formed on their equipment, taking into account the physical or chemical treatment operations specific thereto.